Hybrid and electric vehicles in which the conventional internal combustion engine has been replaced with alternative propulsion technologies must use an alternative source of vacuum pressure for brake-boosted systems. The most common approach to provide vacuum pressure is by way of a stand-alone electric-driven vacuum pump. Vacuum pumps, however, are known to generate objectionable noise.
A common type of conventional vacuum pump includes a pump housing which contains a chamber in which reciprocates a piston. A suction check valve is provided between a suction conduit and the inlet of the chamber. A discharge check valve is provided between the outlet of the chamber and an exhaust conduit. Each of the suction check valve and the discharge check valve typically includes a valve body through which extends multiple discharge port openings of equal size or diameter. A flexible valve disk is positioned adjacent to the valve body.
In operation of the suction check valve, as the piston is displaced in one direction in the chamber, air flows from the suction conduit through the discharge port openings and against the valve disk, respectively, of the suction check valve to dislodge the valve disk from the valve body and thereby facilitate flow of the air into the chamber. This generates negative pressure in the suction conduit. At this time, the exhaust check valve remains closed as the valve disk engages the valve body of the exhaust check valve. As the piston is subsequently displaced in the opposite direction in the chamber, the suction check valve closes as the valve disk engages the valve body of the suction check valve. The air flows from the chamber through the open exhaust check valve and is discharged from the exhaust conduit. Due to the uniform diameter of the discharge port openings in the valve body of each of the suction check valve and the exhaust check valve, a noise is generated when each check valve closes as the valve disk engages the valve body.